Subterranean wells are typically cased with a string of steel wellbore tubulars (piping) coupled end-to-end and cemented in place in the wellbore. The casing string is intended to prevent the wellbore from deterioration and also provides a conduit for produced hydrocarbons. It is often necessary to precisely locate one or more of the joints at which adjacent wellbore tubulars are coupled (e.g., a threaded joint where the male end of one tubular is threadably coupled to the female end of an adjacent tubular). This need arises, for example, when it is necessary to sidetrack an existing well.
Numerous downhole tools and methods are known in the art for locating casing joints (such tools are referred to herein—and commonly in the art—as casing collar locators or sometimes more simply as locators). For example, conventional casing collar locators typically rely on the generation of a strong magnetic field using either a permanent magnet or an electromagnet deployed on the locator. As the locator is moved past a collar, the flux density of the magnetic field changes due to the increased thickness of the collar. The change in magnetic flux produces an electric signal that is transmitted to the surface via a conventional wireline.
Such conventional casing collar locators suffer from many known operational disadvantages. For example, conventional locators are not particularly sensitive to changes in the casing string and thus tend to exhibit a low signal to noise ratio. Moreover, as a result of their insensitivity to changes in the casing string, conventional locators are essentially “collar” locators (rather than “joint” locators) and are generally not able to reliably detect other types of casing joints, such as box and pin joints (also referred to in the art as flush joints). Furthermore, and also as a result of their insensitivity to changes in the casing string, conventional locators are generally only reliable when they are moved rapidly through the wellbore. If the locator is moved too slowly, the changes in signal indicative of the presence of a collar may be too gradual to be conclusively recognized.
More recently, other casing collar locators have been developed to address some of the aforementioned drawbacks with conventional casing collar locators. For example, U.S. Pat. No. 5,720,345 to Price et al. and U.S. Pat. Nos. 6,411,084 and 6,815,946 to Yoo disclose downhole tools that detect magnetic fields indicative of the presence of the casing joints. Price et al. discloses a magnetometer based wireline tool. The magnetic field is continuously measured while the tool is moved through the casing string. It is further disclosed that the magnetic field inside the casing changes at a maximum rate at the casing joint as the wireline tool is moved past the joint. Yoo discloses a wireline tool including a giant magnetoresistive sensor intended to detect perturbations in the earth's magnetic field caused by anomalies in the casing string. Such anomalies are disclosed to include gaps between casing tubulars, enlarged casing wall thickness due to external collars, and air gaps in the threads of a casing joint. Yoo also discloses detection of other anomalies not associated with casing joints such as perforations and damage to the casing string.
The aforementioned devices overcome some of the limitations of conventional casing collar locators, in particular, in that they are more sensitive to changes in the casing string. Despite the advances disclosed by Price et al. and Yoo, the use of such casing collar locators is disadvantageous for certain applications. For example, casing collar locators known in the art (including those described above) are wireline tools. As such, their use requires a separate wireline run into the borehole to determine the locations of various casing joints. As described in more detail below, a typical sidetracking operation includes running a wireline casing collar locator into the borehole to determine the location of a particular casing joint and to set a bridge plug. Only after the bridge plug has been set and the wireline tool removed from the borehole can the drill string and accompanying whipstock be lowered into the borehole. Sidetracking operations including wireline runs are known in the art to be both time consuming and expensive.
Therefore there exists a need for an improved method for locating casing joints. In particular, there exists a need for a measurement while drilling based method for locating casing joints that does not require a separate wireline run into the borehole.